The present invention relates to a method for estimating the pressure in a wheel brake cylinder, particularly in connection with a vehicle stability control system, and a control unit for carrying out the method.
In vehicle stability control systems, the pressure in the respective wheel brake cylinders is determined as accurately as possible to carry out control as effectively and quietly as possible. A method for estimating pressures of this type in connection with a vehicle stability control system is known from U.S. Pat. No. 5,154,491. Reference is also made to International Published Patent Application No. 92/05986 in this regard. This publication describes an anti-lock brake system in which multiple measured and estimated quantities are used to determine a reference slip value, which is compared to an actual slip value. The difference is converted, along with other quantities, to a reference brake pressure, which is then converted to a valve control time.
The known methods have proven to be relatively expensive in practice.
An object of the present invention is therefore to provide a method that can be used to easily estimate a pressure prevailing in a wheel brake cylinder, i.e., a pressure change (or pressure stroke) prevailing there, as well as to provide a control unit for carrying out the method.
According to the present invention, wheel brake cylinder pressures that occur, particularly in connection with a vehicle stability control system, can be easily and reliably estimated. Likewise, the method according to the present invention can be implemented, i.e., can be carried out with a corresponding control unit, in all systems and controllers that affect braking to ensure handling stability and/or safety of a vehicle.
Particularly in the case of vehicle stability control systems that meet high comfort and safety standards, slip is controlled by driving intake valves with PWM output stages (known as LMV valves, quiet or linear solenoid valves), where the pressure building up in the wheel brake cylinder is estimated with a high degree of accuracy. The method described according to the present invention, i.e., the corresponding algorithm, is especially well suited for this purpose. The method described can be used to provide a largely modular design of the software for the vehicle stability control algorithm, particularly also in connection with LMV valves, by using the wheel valve control time as an interface quantity.
The pressure is suitably estimated in the wheel brake cylinder based on a pressure determined in the preceding controller cycle and the pressure stroke calculated in an instantaneous controller cycle.
A current rise and fall time deltaT of the valve is preferably taken into account in the control time specified by the control unit and/or in the calculation of the effective control time.
According to a further preferred embodiment, a presettable minimum pressure difference across the valve is determined as a condition for carrying out the method according to the present invention. This measure prevents serious relative errors, which can occur with minimal pressure differences across the valve, from leading to an inaccurate pressure estimate.
The method according to the present invention and a control unit for carrying out the method are especially well suited for controlling a normally open intake valve that actuates a wheel brake cylinder.